Thin film optical sensors have been used based on an optical sensor measuring changes in a thin sensing film for detecting and measuring physical quantities such as pH, metal-ion, and toxic gas levels. While different optical sensors exist, certain optical sensors include the use of a photometer, which is a device used to measure the optical properties of a thin film which is responsive to a particular physical characteristic or quantity to be measured. In other words, the optical properties of the thin sensing film are responsive to the chemical properties of the physical quantities being measured. For example, the thin film can be responsive to pH, and changes in the optical properties of the film resulting from changes in the pH level are measured by the photometer whose output is calibrated in terms of pH.
Important application areas for such sensors can be found in environmental and clinical applications where there is a need for reliable, low-cost and portable sensors. However, due at least in part to the complexities of realizing analytical instrumentation to meet the demands of the above applications, the foregoing need has not been adequately satisfied. Indeed, only few reports describing such instrumentation have appeared, such as R. Smardzewski, "Multi-Element Optical Waveguide Sensor: General Concept and Design", Talanta, Vol. 35, No. 2, pp. 95-101 (1988), and A. Guthrie et al., "Solid-State Instrumentation For Use With Optical-Fibre Chemical-Sensors", Talanta, Vol. 35, No. 2, pp. 157-159 (1988).
An important focus of the prior attempts has been the provision of low-cost, solid state components for the optical sensor portion of the instrument, which have included the use of light emitting diodes (LEDS) as light sources and photodiodes as detectors. The LED has an additional advantage of producing light at only single defined wavelengths although at variable intensities. In the Smardzewski article cited above, for example, a multi-element optical waveguide sensor for detection and identification of gaseous or liquid mixtures was disclosed. For each component or element to be detected and measured, an optical waveguide such as a cylindrical glass capillary tube was provided. Each optical waveguide was externally coated with a thin film known to react specifically with the particular element to be detected. An LED was then attached to each waveguide, and each waveguide was fiber-coupled to a single photodetector, so that the photodetector provided an output indicative of the level of the element being detected. As is apparent, this sensor operated in a single-wavelength mode, i.e., a single LED provided a light output at a particular wavelength for each waveguide. However, optical sensors such as these which operate in a single-wavelength mode experience calibration problems, due in part to variations in the LED output intensity due to time, temperature, and life of the LEDS, and the degradation of the sensing films. As would be expected, these calibration problems lead to inaccuracy and instability in the sensor response.
In addition to optical sensors utilizing single-wavelength mode operation, two-wavelength schemes have been developed. For example, in the Guthrie et al. article cited above, a two-wavelength scheme was employed. There, an optical fiber pH sensor was incorporated with a solid state instrument including two LEDs and a photodiode detector. One LED provided a measuring wavelength, while the second LED provided a near-infrared "reference wavelength". The respective wavelengths of light were transmitted to a sensor probe on separate optical fibers and the signal intensity was measured at each wavelength by the single detector. Because the light emitted at the reference wavelength was not absorbed by the indicator reagent of the sensor probe, the reflected light intensity at the reference wavelength was independent of indicator state. The signal intensities at the measuring and reference wavelengths were then divided in order to provide a measurement dependent only on the indicator state. Thus, the reference wavelength was utilized to compensate for changes in the signal intensity due to non-chemical causes, such as fiber-bending intensity losses or intensity changes at the fiber connections. However, similar to single-wavelength mode sensors, this two-wavelength device used two completely independent optical sources for illuminating the sensor, and did not compensate for variations in the LED output intensities due to time, temperature, and life of the LEDS, or for variations due to degradation of the sensing film.
In addition to light source output fluctuations, the optical properties of the thin sensing films such as the concentration of the indicator, and the ability of the films to sense the measured physical quantities can change over time resulting in degradation of the sensing films, which further contributes to long-range stability problems. Attempts have been made to combat the long-term stability problems with respect to the optical characteristics of thin sensing films by, for example, regenerating the reagent associated with the film, using controlled release films, and the like. However, none of these techniques have provided optical sensing devices with the desired long-term stability and minimal recalibration requirements.
Another problem not fully addressed by prior developments is that many of the targeted applications demand extreme miniaturization of both the optical and electrical components of the optical sensors. Furthermore, in instances where implantation into a biological host is required for measuring physiological parameters such as blood pH, sodium, potassium, or calcium, biocompatibility of the optical sensor components is of considerable importance. Thus, as is apparent, the development of reliable, low-cost, and long-term optical sensors for environmental and clinical applications has not yet been achieved by the previous developments discussed above.